[media] em28xx: make em28xx-video to be a separate module

Now that all analog-specific code are at em28xx-video, convert
it into an em28xx extension and load it as a separate module.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Tested-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>